Photodetectors are electronic devices engineered to detect light. This light can be visible light or invisible light, such as infrared or ultraviolet light. Detecting and quantifying light in the visible range allows to take pictures of what we see. Detecting and quantifying light in the invisible allows us to explore and, ideally, take pictures of what we do not see.
Photodetectors based on semiconductors can be classified in different classes according to the physical principle being exploited in order to detect light.
In the “photoconductor” class, light is detected by measuring the change in the electrical conductivity of the semiconductor caused by the absorption of light. The first photoconductor was demonstrated by T. W. Case in 1920. See T. W. Case, “Thalofide Cell—A New Photo-Electric Substance,” Physical Review 15, 289 (1920). Some recent photoconductors utilize an intraband absorption in quantum dots or quantum wells. See, e.g., S. Chakrabarti et al., “High-performance, long-wave (˜10.2 μm) InGaAs/GaAs quantum dot infrared photodetector with quaternary In0.21Al0.21Ga0.58As capping,” Appl. Phys. Lett. 99, 181102 (2011); and B. F. Levine, “Quantum-well infrared photodetectors,” J. Appl. Phys. 74, R1 (1993).
In the “photodiode” class, light is detected via the photocurrent induced in a p-n or metal-semiconductor junction. One of the early studies of photodetector theory is attributed to W. W. Gartner, who examined such devices in the 1950s. See, e.g., W. W. Gartner, “Depletion-layer Photoeffects in Semiconductors,” Physical Review 116, 84 (1959).
In the “capacitive” class of photodetectors, detection takes place by exploiting the sensitivity of the capacitance of the given semiconductor device to light. An example of the latest is the metal-oxide-semiconductor capacitor (MOS-C) used in charge-coupled devices (CCDs) which was first mentioned by Boyle and Smith in 1970. See W. S. Boyle and G. E. Smith, “Charge Coupled Semiconductor Devices,” Bell System Technical Journal 49, 587 (1970).